Metal oxide containing multiple dopants and methods of preparing same

ABSTRACT

The present invention relates to metal oxides containing multiple dopants. The metal oxides have the formula:  
     LiM y-x [A] x O z  or M y-x [A] x O z ,  
     wherein M is a transition metal,  
           0   &lt;   x     ,   y   ,       [   A   ]     =     ∑     i   =   l     n                                  
 
     w i B i  wherein B i  is an element used to replace the transition metal M and w i  is the fractional amount of element B i  in the total dopant combination such that  
         ∑     i   =   l     n                              
 
     w i   =1 , n is the total number of dopant elements used and is a positive integer of two or more, wherein the fractional amount w i  of dopant element B i  is determined by the relationship  
       ∑     i   =   l     n                 
 
     w i E i =the oxidation state of the transition metal M±0.5, E i  is the oxidation state of dopant B i  in the final product LiM y-x [A] x O z  or M y-x [A] x O z  the dopant elements B i  are cations in the intercalation compound, and the ratio of Li to O in the intercalation compound is not smaller than the ratio of Li to O in the undoped compound LiM y O z  or M y O z . The present invention also includes methods of preparing same and specific embodiments of same.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of allowed U.S.application Ser. No. 10/165,023, filed Jun. 7, 2002, which is acontinuation application of abandoned U.S. application Ser. No.08/954,372, filed Oct. 20, 1997, which is related to commonly ownedcopending provisional application Ser. No. 60/046,570, filed May 15,1997, and copending provisional application Ser. No. 60/046,571, filedMay 15, 1997. The benefit of the earlier filing dates of each of theseapplications is claimed under 35 U.S.C. § 119(e) and each of theapplications and patents issuing therefrom are incorporated herein intheir entireties by reference.

FIELD OF THE INVENTION

[0002] This invention relates to metal oxide compounds and topreparation methods thereof. More specifically, this invention relatesto doped metal oxide insertion compounds for use in lithium andlithium-ion batteries.

BACKGROUND OF THE INVENTION

[0003] Metal oxides such as lithium metal oxides have found utility invarious applications. For example, lithium metal oxides have been usedas cathode materials in lithium secondary batteries. Lithium and lithiumion batteries can be used for large power applications such as forelectric vehicles. In this specific application, lithium or lithium ioncells are put in series to form a module. In the event that one or moreof the cells in the module fails, the rest of the cells becomeovercharged resulting possibly in explosion of the cells. Therefore, itis important that each cell is individually monitored and protectedagainst overcharging.

[0004] The most attractive materials for use as cathode materials forlithium ion secondary batteries have been LiCoO₂, LiNiO₂, and LiMn₂O₄.However, although these cathode materials are attractive for use inlithium ion secondary batteries, there are definite drawbacks associatedwith these materials. One of the apparent benefits for using LiNiO₂ andLiCoO₂ as cathode materials is that those lithium metal oxides have atheoretical capacity of 275 mA·hr/g. Nevertheless, the full capacity ofthese materials cannot be achieved in practice. In fact, for pure LiNiO₂and LiCoO₂, only about 140-150 mA·hr/g can be used. The further removalof lithium by further charging (overcharging) the LiNiO₂ and LiCoO₂material degrades the cycleability of these materials by moving nickelor cobalt into the lithium layers. Furthermore, the further removal oflithium causes exothermic decomposition of the oxide in contact with theorganic electrolyte under heated conditions which poses safety hazards.Therefore, lithium ion cells using LiCoO₂ or LiNiO₂ are typicallyovercharge protected.

[0005] LiCoO₂ and LiNiO₂ have additional disadvantages when used inlithium ion batteries. Specifically, LiNiO₂ raises safety concernsbecause it has a sharper exothermic reaction at a lower temperature thanLiCoO₂. As a result, the charged end product, NiO₂, is unstable and canundergo an exothermic decomposition reaction releasing O₂ (Dahn et al,Solid State Ionics, Vol. 69, 265 (1994)). Accordingly, pure LiNiO₂ isgenerally not selected for use in commercial lithium-ion batteries.Additionally, cobalt is a relatively rare and expensive transitionmetal, which makes the positive electrode expensive.

[0006] Unlike LiCoO₂ and LiNiO₂, LiMn₂O₄ spinel is believed to beovercharge safe and is a desirable cathode material for that reason.Nevertheless, although cycling over the full capacity range for pureLiMn₂O₄ can be done safely, the specific capacity of LiMn₂O₄ is low.Specifically, the theoretical capacity of LiMn₂O₄ is only 148 mA·hr/gand typically no more than about 115-120 mA·hr/g can be obtained withgood cycleability. The orthorhombic LiMnO₂ and the tetragonallydistorted spinel Li₂Mn₂O₄ have the potential for larger capacities thanis obtained with the LiMn₂O₄ spinel. However, cycling over the fullcapacity range for LiMnO₂ and Li₂Mn₂O₄ results in a rapid capacity fade.

[0007] Various attempts have been made to either improve the specificcapacity or safety of the lithium metal oxides used in secondary lithiumbatteries. For example, in an attempt to improve the safety and/orspecific capacity of these lithium metal oxides, these lithium metaloxides have been doped with other cations. For example, lithium andcobalt cations have been used in combination in lithium metal oxides.Nevertheless, although the resulting solid solution LiNi_(1-x)Co_(x)O₂(0×1) may have somewhat improved safety characteristics over LiNiO₂ andlarger useful capacity below 4.2 V versus Li than LiCoO₂, this solidsolution still has to be overcharge protected just as LiCoO₂ and LiNiO₂.

[0008] One alternative has been to dope LiNiO₂ with ions that have noremaining valence electrons thereby forcing the material into aninsulator state at a certain point of charge, and therefore protectingthe material from overcharge. For example, Ohzuku et al (Journal ofElectrochemical Soc., Vol. 142, 4033 (1995)) describe that the use ofAl³⁺ +s a dopant for lithium nickelates (LiNi_(0.75)Al_(0.25)O₄) canproduce improved overcharge protection and thermal stability in thefully charged state as compared to LiNiO₂. However, the cycle lifeperformance of this material is unknown. Alternatively, U.S. Pat. No.5,595,842 to Nakare et al. demonstrates the use of Ga³⁺ instead of Al³⁺.In another example, Davidson et al (U.S. Pat. No. 5,370,949)demonstrates that introducing chromium cations into LiMnO₂ can produce atetragonally distorted spinel type of structure which is air stable andhas good reversibility on cycling in lithium cells.

[0009] Although doping lithium metal oxides with single dopants has beensuccessful in improving these materials, the choice of single dopantswhich can be used to replace the metal in the lithium metal oxide islimited by many factors. For example, the dopant ion has to have theright electron configuration in addition to having the right valency.For example, Co³⁺, Al³⁺, and Ga³⁺ all have the same valency but Co³⁺ canbe oxidized to Co⁴⁺ while Al³⁺, and Ga³⁺ cannot. Therefore doping LiNiO₂with Al or Ga can produce overcharge protection while doping with cobaltdoes not have the same effect. The dopant ions also have to reside atthe correct sites in the structure. Rossen et al (Solid State IonicsVol. 57, 311 (1992)) shows that introducing Mn into LiNiO₂ promotescation mixing and therefore has a detrimental effect on performance.Furthermore, one has to consider the ease at which the doping reactioncan be carried out, the cost of the dopants, and the toxicity of thedopants. All of these factors further limit the choice of singledopants.

SUMMARY OF THE INVENTION

[0010] The present invention uses multiple dopants to replace thetransition metal M in lithium metal oxides and metal oxides having theformula LiM_(y)O_(z) or M_(y)O_(z) to have a collective effect on theseintercalation compounds. As a result, the choice of dopants is notlimited to elements having the same valency or site preference in thestructure as the transition metal M, to elements having only a desiredelectron configuration, and to elements having the ability to diffuseinto LiM_(y)O_(z) or M_(y)O_(z) under practical conditions. The use of acarefully chosen combination of multiple dopants widens the choices ofdopants which can be used in the intercalation compounds and also canbring about more beneficial effects than a single dopant. For example,the use of multiple dopants can result in better specific capacity,cycleability, stability, handling properties and/or cost than has beenachieved in single dopant metal oxides. The doped intercalationcompounds of the invention can be used as cathode materials inelectrochemical cells for lithium and lithium-ion batteries.

[0011] The doped lithium metal oxides and doped metal oxides of theinvention have the formula:

LiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z),

[0012] wherein M is a transition metal,${{0 < x},y,{\lbrack A\rbrack = \sum\limits_{i = l}^{n}}}\quad$

[0013] w_(i)B_(i) wherein B_(i) is an element used to replace thetransition metal M and w_(i) is the fractional amount of element B_(i)in the total dopant combination such that$\sum\limits_{i = l}^{n}\quad$

[0014] w_(i)=1, n is the total number of dopant elements used and is apositive integer of two or more, the fractional amount w_(i) of dopantelement B_(i) is determined by the relationship$\sum\limits_{i = l}^{n}$

[0015] w_(i)E_(i)=the oxidation state of the replaced transition metalM±0.5, E_(i) is the oxidation state of dopant B_(i) in the final productLiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z) the dopant elements B_(i)are cations in the intercalation compound, and the ratio of Li to O inthe doped intercalation compound is not smaller than the ratio of Li toO in the undoped compound LiM_(y)O_(z) or M_(y)O_(z). Typically, M isselected from Co, Ni, Mn, Ti, Fe, V and Mo and the dopant elements B_(i)are any elements other than M having a Pauling's electronegativity notgreater than 2.05 or Mo.

[0016] In one preferred embodiment of the invention, the intercalationcompound has a formula LiM_(y-x)[A]_(x)O_(z) wherein M is Ni or Co andthe dopant elements B_(i) include Ti⁴⁺ and Mg²⁺. The formulasLiNi_(1-x)Ti_(a)Mg_(b)O₂ and LiCo_(1-x)Ti_(a)Mg_(b)O₂ can also be usedto describe these intercalation compounds wherein x=a+b and x ispreferably in the range from greater than 0 to about 0.5. Morepreferably, a is approximately equal to b and b is no smaller than a forthese intercalation compounds. The dopant elements B_(i) can furtherinclude other cations or have the formula LiM_(y-x)[A]_(x)O_(z) whereinM is Ni or Co, y=1, z=2, and the dopant elements B_(i) include Ti⁴⁺,Mg²⁺ and Li⁺ cations.

[0017] The present invention also includes a method of preparing a dopedintercalation compound having the formula LiM_(y-x)[A]_(x)O_(z) orM_(y-x)[A]_(x)O_(z). Source compounds containing M, [A] and optionallyLi are mixed to provide a stoichiometric relationship between M, [A] andLi corresponding to the formula LiM_(y-x)[A]_(x)O_(z) orM_(y-x)[A]_(x)O_(z) wherein M is a transition metal,${0 < x},y,{\lbrack A\rbrack = {\sum\limits_{i = l}^{n}\quad {w_{i}B_{i}}}}$

[0018] wherein B_(i) is an element used to replace the transition metalM and w_(i) is the fractional amount of element B_(i) in the totaldopant combination, n is the total number of dopant elements used and isa positive integer of two or more, the fractional amount w_(i) of dopantelement B_(i) is determined by the relationship:$\sum\limits_{i = l}^{n}$

[0019] w_(i)E_(i)=the oxidation state of the replaced transition metalM±0.5, E_(i) is the oxidation state of dopant B_(i) in the final productLiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z), the dopant elements B; areselected to be cations in the intercalation compound, and the ratio ofLi to O in the doped intercalation compound is not smaller than theratio of Li to O in the undoped compound LiM₄O₂ or M_(y)O_(z). Thecations for the intercalation compound can each be supplied fromseparate source compounds or two or more of the cations can be suppliedfrom the same source compounds. The mixture of source compounds is firedat a temperature between 500° C. and 1000° C. in the presence of oxygento produce the intercalation compound and preferably cooled in acontrolled manner to produce a doped intercalation compound suitable foruse as a cathode material for electrochemical cells for lithium andlithium-ion batteries.

[0020] These and other features and advantages of the present inventionwill become more readily apparent to those skilled in the art uponconsideration of the following detailed description which describes boththe preferred and alternative embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is an x-ray diffraction pattern study for four differentintercalation compounds produced according to the present invention.

[0022]FIG. 2 is an x-ray diffraction pattern study for two differentintercalation compounds produced according to the invention anddemonstrating the desirability of maintaining valency in theintercalation compound.

[0023]FIG. 3 is a voltage profile for three slow cycles between 3.0 V to5.0 V for a fresh electrochemical cell containing an intercalationcompound produced according to a preferred embodiment of the presentinvention.

[0024]FIG. 4 is a voltage profile between 3.0 V and 4.5 V for thecompound of FIG. 3 after the three slow cycles and demonstrating thecycleability of the compound.

[0025]FIG. 5 is a graph of discharge capacity versus cycle number forthe same compound as FIG. 4 and following the same cycling pattern asFIG. 4.

[0026]FIG. 6 is a graph of discharge capacity versus cycle number for anelectrochemical cell containing the same intercalation compound testedin FIGS. 3-5 and following the same cycling pattern as in FIGS. 3-5.

[0027]FIG. 7 is a differential scanning calorimetry (DSC) scan of threeof the intercalation compounds tested in FIG. 1 and of LiNiO₂.

[0028]FIG. 8 is an x-ray diffraction pattern for an intercalationcompound produced in accordance with the present invention both beforeand after acid treatment of the intercalation compound.

[0029]FIG. 9 is an x-ray diffraction pattern for an intercalationcompound produced according to another preferred embodiment of thepresent invention.

[0030]FIG. 10 is an x-ray diffraction pattern for an intercalationcompound produced according to yet another preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which preferredembodiments of the invention are shown. This invention can, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Inthe following description, the invention is described primarily withrespect to LiNiO₂. Nevertheless, the present invention should not belimited thereto and can be used with various intercalation compoundsincluding a wide range of lithium metal oxides and metal oxidesincluding, e.g., LiMnO₂, LiCoO₂, Li₂Mn₂O₄, LiMn₂O₄, MnO₂, and V₂O₅.

[0032] The doped lithium metal oxides and doped metal oxides of theinvention have the formula:

LiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z),

[0033] wherein M is a transition metal,

[0034] 0<x, y, $\lbrack A\rbrack = \sum\limits_{i = l}^{n}$

[0035] w_(i)B_(i) wherein B_(i) is an element used to replace thetransition metal M and w_(i) is the fractional amount of element B_(i)in the total dopant combination and therefore$\sum\limits_{i = l}^{n}\quad$

[0036] w_(i)=1, n is the total number of dopant elements used and is apositive integer of two or more, wherein the fractional amount w_(i) ofdopant element B_(i) is determined by the relationship${{\sum\limits_{i = l}^{n}{w_{i}E_{i}}} = {{{the}\quad {oxidation}\quad {state}\quad {of}\quad {the}\quad {replaced}\quad {transition}\quad {metal}\quad M} \pm 0.5}},$

[0037] w_(i)E_(i)=the oxidation state of the replaced transition metalM±0.5, Es is the oxidation state of dopant B_(i) in the final productLiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z), the dopant elements B_(i)are cations in the intercalation compound, and the ratio of Li to O inthe intercalation compound is not smaller than the ratio of Li to O inthe undoped compound LiM_(y)O_(z) or M_(y)O_(z).

[0038] The doped lithium metal oxide and doped metal oxide compounds ofthe invention can be described as intercalation or insertion compounds.The preferred doped metal oxide compound is a lithium metal oxideintercalation compound having the formula LiM_(y-x)[A]_(x)O_(z). In theintercalation compounds of the invention, M is typically selected fromCo, Ni, Mn, Ti, Fe, V and Mo. The dopant elements B_(i) are any elementsother than M having a Pauling's electronegativity not greater than 2.05or Mo (i.e., if M is not Mo). In other words, the dopant elements B_(i)are elements other than M selected to be cations in the intercalationcompound. The dopant elements B_(i) preferably include no more than oneelement from the Groups IIIB and IVB (e.g. Al and Si). Furthermore, thedopant elements B_(i) are selected so that$\sum\limits_{i = l}^{n}{w_{i}E_{i}}$

[0039] w_(i)E_(i) preferably approaches the oxidation state of thetransition metal M and more preferably is equal to the oxidation stateof the transition metal M in the undoped metal oxide LiM_(y)O_(z) orM_(y)O_(z).

[0040] The molar quantity of oxygen, z, in the intercalation compoundsof the invention is such that the metal oxide is a stable, single phasemetal oxide compound. Furthermore, as described above, the molarquantity of oxygen is such that the ratio of Li to O in the dopedintercalation compound is not smaller than the ratio of Li to O in theundoped compound LiM_(y)O_(z) or M_(y)O_(z). Accordingly, the transitionmetal M is replaced with the dopant ions and the lithium is notsubstituted to maximize the specific capacity of the intercalationcompound.

[0041] Various combinations of multiple dopants can be used according tothe present invention in place of single dopants used in conventionalintercalation compounds having the formula LiM_(y-x)A_(x)O_(z) orM_(y-z)A_(x)O_(z). For example, in LiNi_(1-x)A_(x)O₂ intercalationcompounds, the dopants Al³⁺ and Ga³⁺ conventionally described to replaceNi³⁺, can be replaced with 0.5Ti⁴⁺+0.5Mg²⁺ and still maintain the samecharge balance in the compound. Instead of having LiNi_(1-x) ³⁺Alx³⁺O₂,one has LiNi_(1-x) ³⁺Ti_(x/2) ⁴⁺Mg_(x/2) ²⁺O₂. Since it is believed thatNi³⁺ can only be oxidized to Ni⁴⁺, only (1-x) Li per formula unit can beremoved. Like Al³⁺ and Ga³⁺, both Ti⁴⁺ and Mg²⁺ have no remainingvalence electrons. Therefore, when the material composition reachesLi_(x)Ni_(1-x) ⁴⁺Ti_(x/2) ⁴⁺Mg_(x/2) ²⁺O₂ no more lithium can be removedand the voltage will simply increase sharply. Thus overcharge protectionis achieved. In addition, Ti⁴⁺ and Mg²⁺ bind with oxygen more stronglythan Ni⁴⁺ and therefore LixNi is more stable than NiO₂. Accordingly,single dopant Al³⁺ or Ga³⁺ or Ni³⁺ itself in LiNiO₂ can be replaced by[0.5Mg²⁺+0.5Ti⁴⁺] or even [0.667 Mg²⁺+0.333 V⁵⁺] and other combinationsof cations to achieve the overcharge protection and at the same timetaking the benefit of the latter's larger binding energy with oxygen toachieve better material stability. Furthermore, LiCoO₂ can be doped inthe manner described above with respect to LiNiO₂.

[0042] In one preferred embodiment of the invention, the intercalationcompound has a formula LiM_(y-x)[A]_(x)O_(z) wherein M is Ni or Co andthe dopant elements B_(i) include Ti⁴⁺ and Mg²⁺. The formulasLiNi_(1-x)Ti_(a)Mg_(b)O₂ and LiCo_(1-x)Ti_(a)Mg_(b)O₂ wherein x=a+b andx is preferably in the range from greater than 0 to about 0.5, can alsobe used to describe these intercalation compounds which have a hexagonallayered crystal structure. More preferably, a is approximately equal tob and b is no smaller than a for these intercalation compounds. It hasbeen discovered that these materials, when used as the positiveelectrodes in lithium secondary electrochemical cells, have largespecific capacities, are safer than LiNiO₂, and have goodcycleabilities. The balance between having a large capacity and athermally benign material can be achieved by adjusting x.

[0043] The use of Ti and Mg at the same time imposes intrinsicovercharge protection on the intercalation compounds and improves thesafety of the material while maintaining good cycleability at largecapacities. For example, it is believed that Ti and Mg have the formTi⁴⁺ and Mg²⁺ in LiNi_(1-x)Ti_(y)Mg_(z)O₂ because the energies of Mg 2selectrons are higher than Ti 3d electrons which in turn are higher thanNi 3d electrons (Yeh et al, Atomic Data and Nuclear Data Tables Vol. 32,1-155 (1985)). It can be shown that the oxidation state of nickel equals3 when a=b so that the material can be written as Li⁺Ni_(1-x) ³⁺(Ti)_(y)⁴⁺Mg_(y) ²⁺O₂ where y=a/2. Since there are no remaining valenceelectrons in either Ti⁴⁺ or Mg²⁺, only (1-x) Li per formula unit can beremoved and therefore overcharge protection is achieved intrinsically.In other words, the charge will stop when all the Ni³⁺ are oxidized toNi⁴⁺ and the fully charged material is Li_(x) ⁺Ni_(1-x) ⁴⁺Ti_(y)⁴⁺Mg_(y) ²⁺O₂. Also, the material is believed to be more stable againstdecomposition in the fully charged state than LiNiO₂. This is becauseTi⁴⁺ and Mg²⁺ bind oxygen more strongly than Ni⁴⁺, as evidenced by thefact that TiO₂ and MgO are very stable oxides and NiO₂ is not. Thisstability improves the safety of the material under overchargeconditions in lithium ion electrochemical cells. Because the averageoxidation state of nickel is less than 3 in LiNi_(1-x)Ti_(a)Mg_(b)O₂when b<a, it is preferred that b≧a because Ni²⁺ ions tend to migrate tothe lithium layers, causing diffusion problems for lithium duringelectrochemical charge and discharge. Furthermore, it is preferred thatb is not much greater than a because the oxidation state of nickel willapproach 4 which makes it difficult to formulate single phaseintercalation compounds. Therefore, the ratio of b:a is preferablybetween about 1 and about 1/x.

[0044] In the preferred embodiment described above, wherein M is Ni, thedopant elements B_(i) can further include other cations such as cobaltcations. In addition, Li⁺ ions can be used as a dopant with otherdopants such as Ti⁴⁺, either alone or in combination with Mg²⁺. In otherwords, intercalation compounds can have the formulaLiM_(y-x)[A]_(x)O_(z) wherein M is Ni, y=1, z=2, and the dopant elementsB_(i) include Ti⁴⁺ and Li⁺ cations. In such an embodiment, the [0.5Ti⁴⁺+0.5 Mg²⁺] described in the preferred embodiment above can bereplaced by [0.667 Ti⁴⁺+0.333 Li+]. Alternatively, the intercalationcompound can also include Mg²⁺ as a dopant such that the intercalationcompound has a formula LiM_(y-x)[A]_(x)O_(z) wherein M is Ni, y=1, z=2,and the dopant elements B_(i) include Ti⁴⁺, Mg²⁺ and Li⁺ cations. Insuch an embodiment, the [0.5 Ti⁴⁺+0.5 Mg²⁺] described in the preferredembodiment above can be replaced by [0.6 Ti⁴⁺+0.2 Mg²⁺+0.2 Li⁺]. As willbe recognized by those skilled in the art, the above formulas can bealtered when Li⁺ is used as a dopant, e.g.,LiM_(y-x)Ti_(0.6x)Mg_(0.2x)Li_(0.2x)O_(z) can also be written asLi_(1+0.2)M_(y-x)Ti_(0.6x)Mg_(0.2x)O_(z) for the latter example.

[0045] The present invention can also be applied to many other types oflithium metal oxide and metal oxide cathode materials. For instance, onecan replace Mn⁴⁺ with 0.4Li++0.6Mo⁶⁺ or 0.25Li++0.75V⁵⁺ in LiMn₂O₄ sothat more Li⁺ ions can be introduced into the octahedral 16d sites toimprove the structural stability without causing significant capacitydecrease.

[0046] The present invention also includes a method of preparing a dopedintercalation compound having the formula LiM_(y-x)[A]_(x)O_(z) orM_(y-x)[A]_(x)O_(z). Source compounds or raw materials containing M, [A]and optionally Li are mixed to provide a stoichiometric relationshipbetween M, [A] and Li corresponding to the formula LiM_(y-x)[A]_(x)O_(z)or My-x[A]_(x)O_(z), wherein M is a transition metal,${0 < {x.y}},{\lbrack A\rbrack = {\sum\limits_{i = l}^{n}{w_{i}B_{i}}}}$

[0047] wherein B_(i) is an element used to replace the transition metalM and w_(i) is the fractional amount of element B_(i) in the totaldopant combination, n is the total number of dopant elements used and isa positive integer of two or more, the fractional amount w_(i) of dopantelement B_(i) is determined by the relationship:${{\sum\limits_{i = l}^{n}{w_{i}E_{i}}} = {{{the}\quad {oxidation}\quad {state}\quad {of}\quad {the}\quad {replaced}\quad {transition}\quad {metal}\quad M} \pm 0.5}},$

[0048] the oxidation state of the transition metal M±0.5, E_(i) is theoxidation state of dopant B_(i) in the final productLiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z), the dopant elements B_(i)are selected to be cations in the doped intercalation compound, and theratio of Li to O in the doped intercalation compound is not smaller thanthe ratio of Li to O in the undoped compound LiM_(y)O_(z) or M_(y)O_(z).The source compounds (raw materials) can be the pure elements but aretypically compounds containing the elements such as oxides or saltsthereof. The cations for the intercalation compound can each be suppliedfrom separate source compounds or two or more of the cations can besupplied from the same source compounds. In addition, the sourcecompounds can be mixed in any desirable order.

[0049] Although the intercalation compounds are preferably prepared by asolid state reactions, it can be advantageous to react the raw materialsusing wet chemistry such as sol-gel type reactions, alone or incombination with solid state reactions. For example, the sourcecompounds comprising M and [A] can be prepared as a solution in asolvent such as water and the M and [A] precipitated out of solution toproduce an intimately mixed hydroxide. The mixed hydroxide can then beblended with a lithium source compound. Typically, the selection ofreaction methods will vary depending on the raw materials used and thedesired end product.

[0050] The mixture once prepared can be reacted to form the lithiummetal oxide or metal oxide. Preferably, the mixture is reacted by firingthe mixture at an elevated temperature between 500° C. and 1000° C. inthe presence of oxygen, e.g., between about 700° C. and 900° C., in asolid state reaction to produce the intercalation compounds. Once themixture has been fired to form the doped lithium metal oxide or metaloxide intercalation compound, the intercalation compound is preferablycooled in a controlled manner to produce an intercalation compoundsuitable for use as a cathode material for electrochemical cells forlithium and lithium-ion batteries.

[0051] In the preferred embodiment described above having the formulaLiNi_(1-x)Ti_(a)Mg_(b)O₂ or LiCo_(1-x)Ti_(a)Mg_(b)O₂, a single phase canbe obtained by the following steps. First, stoichiometric amounts of alithium source compound, a nickel or cobalt source compound, a titaniumsource compound and a magnesium source compound are mixed in any desiredorder to give the desired molar ratio according to the formulaLiNi_(1-x)Ti_(y)Mg_(z)O₂ or LiCo_(1-x)Ti_(a)Mg_(b)O₂. As describedabove, the lithium, nickel (or cobalt), titanium and magnesium can besupplied by separate source compounds or two or more of these cationscan be supplied by a single source compound. For example, TiMgO₃ andNi_(0.75)Ti_(0.25)O are commercially available compounds which cansupply two cations for use in the intercalation compounds of theinvention. The mixture is then fired at a temperature between 700° C.and 900° C., preferably between 750° C. and 850° C., in an atmospherewith a partial pressure of oxygen of at least 20 kPa, preferably about100 kPa. The fired mixture is then cooled in a controlled manner,preferably at a rate of 50C/min or less. The firing temperature and thesoak times are chosen depending on x and the oxygen partial pressure sothat the lithium to Ni_(1-x)Ti_(y)Mg_(z) ratio in the structurepreferably approximates 1:1 and no significant cation mixing betweenlithium and the other metals occurs in the layers. Suitable compoundsfor the invention include a lithium source compound comprising one orany combination of the following: LiOH, LiNO₃, Li₂CO₃, LiCl and LiF; anickel source compound comprising one or any combination of thefollowing: NiO, Ni(NO₃)₂, Ni(OH)₂ and NiCO₃; a cobalt source compoundcomprising one or any combination of the following: CO₃O₄, Co(OH)₂,CoCO₃, Co(NO₃)₂, CoO, and CO₂O₃; a titanium source compound comprisingone or any combination of the following: a titanium source compoundcomprising TiO₂ in one or any combination of the following forms:anatase, rutile and brookite; and a magnesium source compound comprisingone or any combination of the following: Mg(OH)₂, Mg(NO)₃, MgCO₃, MgCland MgO. Also, TiMgO₃ and Ni_(0.75)Ti_(0.25)O can be used as sourcecompounds as described above.

[0052] As mentioned above, in addition to producing the intercalationcompounds of the invention by solid state methods, these compounds canalso be made by wet chemistry methods. For example, Ni, Ti and Mg can beprecipitated simultaneously from a solution containing the threeresulting in an intimately mixed hydroxide. The mixed hydroxide havingthe desired molar ratio according to the formulaLiNi_(1-x)Ti_(a)Mg_(b)O₂ can then be blended with a lithium sourcecompound and fired at a temperature of between 700° C. and 900° C. in anoxygen-containing atmosphere. In such wet chemistry reactions, it is notnecessary to stay at high temperatures for extended periods of time inorder for the Ti and Mg to diffuse uniformly with Ni.

[0053] The present invention will now be described according to thefollowing non-limiting examples.

EXAMPLE 1

[0054] Stoichiometric amounts of LiOH, NiO, TiO₂, and Mg(OH)₂ are mixedand fired at a temperature of 800° C. for 20 hours in an atmosphere withthe oxygen partial pressure close to 100 kPa. The cooling was controlledat 1 D C/min down to 500° C. followed by natural cooling to roomtemperature. FIG. 1 shows the x-ray diffraction (XRD) patterns for 4samples having the following formulas: LiNi_(0.9)Ti_(0.05)Mg_(0.05)O₂,LiNi_(0.8)Ti_(0.1)Mg_(0.1)O₂, LiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂, andLiNi_(0.7)Ti_(0.15)Mg_(0.15)O₂. As shown in FIG. 1, each of thesesamples are in a single phase having a hexagonal layered structure.Samples were also made with nitrate precursors instead of hydroxides.The same single phase materials were obtained.

EXAMPLE 2

[0055] A intercalation compound having the formulaLiNi_(0.75)Ti_(0.15)Mg_(0.10)O₂ was prepared according to the methoddescribed in Example 1. The x-ray diffraction pattern for this sample isillustrated in FIG. 2 along with the x-ray diffraction pattern of theLiNi_(0.75)Ti_(0.125)Mg _(0.125)O₂ prepared in Example 1. As evidencedby the smaller peak ratio between the 003 peak and the 104 peak forLiNi_(0.75)Ti_(0.15)Mg_(0.10)O₂ as compared toLiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂, there is a greater amount of cationmixing in the LiNi_(0.75)Tio. ₁₅Mg_(0.100) ₂ sample than in theLiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ sample. Therefore, it is important tomaintain the amount of Mg²⁺ greater than equal to the amount of Ti⁴⁺ andpreferably equal to the amount of Ti⁴⁺.

EXAMPLE 3

[0056] Electrochemical cells with lithium metal as the anode andcathodes with LiNi_(0.75)Ti_(0.125)Mg_(0.125) 0 ₂ (prepared according toExample 1) as the active material were made and tested. The electrolytewas IM LiPF₆ in a 50/50 volume percent mixture of ethylene carbonate anddimethyl carbonate solvents. Celgard 3501 separators and NRC 2325 coincell hardware were used. The cathode consisted of 85% active material(by weight), 10% super S™ carbon black (available from Chemetals) and 5%polyvinylidene fluoride (PVDF) as a binder polymer, coated on aluminumfoil. Preliminary test results are shown in FIGS. 3-6. The cathode oftest cell 1 contains 9.1 mg active mass ofLiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂. The cell was first cycled with 0.075mA from 3.0 V to 5.0 V three times. The results of this cycling areillustrated in the voltage (V) to specific capacity (mA-hr/g) graph ofFIG. 3. The current corresponds to a rate close to C/20 or 8.2 mA/g ofactive mass. After the first conditioning charge, the voltage curve ofthe subsequent cycles shows very reversible characteristics. As furthershown in FIG. 3, most of the capacity is contained between 3.6 V and 4.4V versus Li. Above 4.4 V, the voltage increases sharply to 5 V whichshows very good overcharge characteristics. The reversible capacity isabout 190 mA-hr/g. After the three slow cycles, the cell was cycledbetween 3.0 V and 4.5 V at a larger current of 0.6 mA. This currentcorresponds to a faster rate of C/3, or 66 mA/g of active material. Asshown in the voltage (V) to time (hr) graph of FIG. 4, very goodreversibility was maintained and the polarization remained small at thehigher charge/discharge rates. The discharge capacity versus cyclenumber for the 3.0-4.5 V cycling in FIG. 4 is shown in FIG. 5 whichdemonstrates the excellent cycleability of the material.

EXAMPLE 4

[0057] A second test cell (test cell 2) containing 16.2 mg active massof LiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ was prepared. The cell was firstcycled between 3.0 and 5.0 V for 11 cycles, and was then switched to3.0-4.5 V cycling. The current for charge and discharge was 0.6 mA. Asshown in the graph of discharge capacity versus cycle number in FIG. 6,the cycleability of the material was excellent.

EXAMPLE 5

[0058] The LiNi_(0.9)Ti_(0.05)Mg_(0.05)O₂, LiNi_(0.8)Ti_(0.1)Mg_(0.1)O₂and LiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ prepared in Example 1 were eachused as the active cathode material for electrochemical cells preparedin the manner described in Example 3 using between 10 mg and 20 mgactive material for each cell. The cells were first conditioning chargedto 5.0 V and discharged to 3.0 V, and then float charged to 4.5 V with0.2 mA current for 40 hours to ensure equilibrium conditions. Thecharged cells were then transferred to a glove box filled with argon andopened. Between 0.1 mg and 1.0 mg of the cathode material from the cellswas removed and hermetically sealed into DSC cells. Each of cellscontained 10-15% of the electrolyte described in Example 3. FIG. 7illustrates the DSC results for the LiNi_(0.9)Ti_(0.05)Mg_(0.05)O₂,LiNi_(0.8)Ti_(0.1)Mg_(0.1)O₂ and LiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ withthe area of interest magnified in the inset. The positive heat flow inFIG. 7 represent heat flowing out of the sample. As demonstrated in FIG.7, the sharp exothermic peak at 220° C. decreases with increasing x forthe formula LiNi_(l-x)Ti_(a)Mg_(b)O₂ (x=a+b) demonstrating the thermalstability and safety advantage associated with the doped intercalationcompounds.

EXAMPLE 6

[0059] LiNi_(0.75)Ti_(0.125)MgO₁₂₅O₂ prepared as described in Example Iwas tested for acid resistance. Twenty grams ofLiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ was placed in 400 ml deionized water.HCl was added until the pH of the solution reached 2 and the solutionwas stirred for 1 hour. The LiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ wasfiltered and washed with deionized water until the wash reached a pH of7. The first liter of the wash was analyzed with inductively coupledplasma (ICP) spectroscopy. About 25% of the total lithium and less than0.5% of the total Ni in the sample was detected in the wash. No Ti andMg were detected in the wash. The washed and filteredLiNi_(0.75)Ti_(0.125)Mg_(0.125)O₂ was vacuum dried and an x-raydiffraction of this compound was performed. As shown in FIG. 8, the acidtreated sample has the same XRD pattern as an untreated sample and thepeaks are still sharp. Therefore, although there is partial delithiation(lithium leaching) under acidic conditions, the basic structuralintegrity of the material is still maintained as evidenced by theminimal loss of the transition metals and the XRD pattern showing thesame structure and crystallinity of the intercalation compound.

EXAMPLE 7

[0060] A doped intercalation compound having the formulaiNi_(0.75)Ti_(0.15)Mg_(0.05)Li_(0.05)O₂ was prepared by firing astoichiometric mixture of LiOH, NiO, TiO₂ and Mg(OH)₂ at 800° C. for 20hours in air, followed by a 1° C./min controlled cooling to 500° C. andnatural cooling to room temperature. FIG. 9 illustrates an x-raydiffraction pattern for this compound. As demonstrated in FIG. 9, theintercalation compound was a single phase compound and as evidenced bythe peak ratio between the 003 peak and 104 peak, there was no cationmixing in the metal layers. Accordingly, although not wishing to bebound by theory, it is believed that for lithium metal oxides having theformula LiNi_(1-x)Ti_(a)Mg_(b)O₂ (x=a+b), if a>b then deficiencies in Mgcan be compensated by excess Li as long as the average oxidation stateof the Ti—Mg—Li dopant combination is still maintained at about 3.

EXAMPLE 8

[0061] A doped intercalation compound having the formulaLiNi_(0.7)Co_(0.1)Ti_(0.1)Mg_(0.1)O₂ was prepared by firing astoichiometric mixture of LiOH, NiO, CO₃O₄, TiO₂ and Mg(OH)₂ at 800° C.for 20 hours in air, followed by a 1° C./min controlled cooling to 500°C. and natural cooling to room temperature. As shown in FIG. 10, thisintercalation compound was predominantly single phase.

[0062] As shown in the examples, the doped lithium metal oxide or metaloxide intercalation compounds can be used in the positive electrode(cathode) of lithium or lithium-ion electrochemical cells and aretypically combined with a carbonaceous material and a binder polymer toform a cathode. The negative electrode can be lithium metal or alloys,or any material capable of reversibly lithiating and delithiating at anelectrochemical potential relative to lithium metal between about 0.0 Vand 0.7 V, and is separated from the positive electrode material in thecell using an electronic insulating separator. Examples of negativeelectrode materials are carbonaceous materials including carbonaceousmaterials containing H, B, Si and Sn, and tin oxides or tin-siliconoxides. The electrochemical cells further include an electrolyte. Theelectrolyte can be non-aqueous liquid, gel or solid and preferablycomprises a lithium salt. Electrochemical cells using the intercalationcompounds of the invention as positive electrode material can becombined for use in large power applications such as for electricvehicles.

[0063] In the present invention, a combination of multiple dopants canbe selected to replace the transition metal M in intercalation compoundsof the formula LiM_(y)O_(z) or M_(y)O_(z) to achieve the same result asa single dopant. As a result, the limits on the choice of single dopantscan be avoided and, at the same time, more beneficial effects can beachieved by using a combination of two or more dopants. Specifically,the use of multiple dopants can result in better specific capacity,cycleability, stability, handling properties and/or cost than has beenachieved in single dopant metal oxides. Furthermore, the multiple dopedintercalation compounds demonstrate good heat and acid stability andtherefore are safe for use as cathode materials in electrochemical cellsfor lithium and lithium ion batteries.

[0064] Many modifications and other embodiments of the invention willcome to the mind of one skilled in the art to which this inventionpertains having the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed is:
 1. A doped intercalation compound having theformula: LiM_(y-x)[A]_(x)O_(z) or M_(y-x)[A]_(x)O_(z), wherein M=atransition metal,${0 < x < y},{\left\{ A \right\} = {\sum\limits_{i = l}^{n}{w_{i}B_{i}}}}$

wherein B_(i) is an element used to replace the transition metal M andw_(i) is the fractional amount of element B_(i) in the total dopantcombination such that ${{\sum\limits_{i = l}^{n}w_{i}} = 1},$

and n=total number of dopant elements B_(i) and is a positive integer oftwo or more; wherein the fractional amount w_(i) of dopant element B_(i)is determined by the following relation:${{\sum\limits_{i = 1}^{n}{w_{i}E_{i}}} = {{{oxidation}\quad {state}\quad {of}\quad {the}\quad {replaced}\quad {transition}\quad {metal}\quad {ion}\quad M} \pm 0.5}};$

 oxidation state of the replaced transition metal ion M±0.5; whereinE_(i) is the oxidation state of dopant B_(i) in the final productLiM_(y-x){A}_(x)O_(z) or M_(y-x){A}_(x)O_(z); wherein the dopantelements B_(i) are cations in the intercalation compound and at leasttwo of the dopant elements Bi have a different oxidation state than theoxidation state of M in the LiM_(y-x){A}_(x)O_(z) orM_(y-x){A}_(x)O_(z); compound; wherein the dopant elements B_(i) includeTi⁴⁺ and Mg²⁺; wherein y and z are values that provide a stable metaloxide compound; and wherein the ratio of Li to O in the dopedintercalation compound is not smaller than the ratio of Li to O in theundoped compound LiM_(y)O_(z) or M_(y)O_(z).
 2. The intercalationcompound of claim 1, wherein the dopant elements B_(i) are any elementsother than M having a Pauling's electronegativity not greater than 2.05or Mo.
 3. The intercalation compound of claim 1, wherein the dopantelements B_(i) include no more than one element from Groups IIIB andIVB.
 4. The intercalation compound of claim 1, wherein the fractionalamount of Ti⁴⁺ is approximately equal to the fractional amount of Mg²⁺.5. The intercalation compound of claim 1, wherein the fractional amountof Mg²⁺ is no smaller than the fractional amount of Ti⁴⁺.
 6. A positiveelectrode for lithium and lithium ions cells comprising an intercalationcompound having the formula: LiM_(y-x)[A]_(x)O_(z) orM_(y-x)[A]_(x)O_(z), wherein M=a transition metal,${0 < x < y},{\left\{ A \right\} = {\sum\limits_{i = l}^{n}{w_{i}B_{i}}}}$

wherein w_(i) is the fractional amount of element B_(i) in the totaldopant combination such that ${{\sum\limits_{i = l}^{n}w_{i}} = 1},$

and n=total number of dopant elements used and is a positive integer oftwo or more; wherein the fractional amount w_(i) of dopant element B_(i)is determined by the following relation:${{\sum\limits_{i = 1}^{n}{w_{i}E_{i}}} = {{{oxidation}\quad {state}\quad {of}\quad {the}\quad {replaced}\quad {transition}\quad {metal}\quad {ion}\quad M} \pm 0.5}};$

 oxidation state of the replaced transition metal ion M±0.5; whereinE_(i) is the oxidation state of dopant B_(i) in the final productLiM_(y-x){A}_(x)O_(z) or LiM_(y-x){A}_(x)O_(z); wherein the dopantelements B_(i) are cations in the intercalation compound and at leasttwo of the dopant elements Bi have a different oxidation state than theoxidation state of M in the LiM_(y-x){A}_(x)O_(z) orM_(y-x){A}_(x)O_(z); compound; wherein the dopant elements B_(i) includeTi⁴⁺ and Mg²⁺; wherein y and z are values that provide a stable metaloxide compound; and wherein the ratio of Li to O in the dopedintercalation compound is not smaller than the ratio of Li to O in theundoped compound LiM_(y)O_(z) or M_(y)O_(z).
 7. The positive electrodeof claim 6, wherein the dopant elements B_(i) are any elements otherthan M having a Pauling's electronegativity not greater than 2.05 or Mo.8. The positive electrode of claim 6, wherein the dopant elements B_(i)include no more than one element from Groups 111B and IVB.
 9. Thepositive electrode of claim 6, wherein the fractional amount of Ti⁴⁺ isapproximately equal to the fractional amount of Mg²⁺.
 10. The positiveelectrode of claim 6, wherein the fractional amount of Mg²⁺ is nosmaller than the fractional amount of Ti⁴⁺.
 11. A method of preparing adoped intercalation compound of the formula LiM_(y-x){A}_(x)O_(z) orM_(y-x){A}_(x)O_(z), comprising the steps of: mixing source compoundscontaining M, {A} and optionally Li to provide a stoichiometricrelationship between M, {A} and Li corresponding to the formulaLiM_(y-x){A}_(x)O_(z) or M_(y-x){A}_(x)O_(z), wherein M is a transitionmetal,${0 < x < y},{\left\{ A \right\} = {\sum\limits_{i = l}^{n}\quad {w_{i}B_{i}}}}$

wherein B_(i) is an element i=1 used to replace the transition metal Mand w_(i) is the fractional amount of element B_(i) in the total dopantcombination, n is the total number of dopant elements and is a positiveinteger of two or more, the fractional amount w_(i) of dopant elementB_(i) is determined by the relationship:${{\sum\limits_{i = l}^{n}\quad {w_{i}E_{i}}} = {{{oxidation}\quad {state}\quad {of}\quad {the}\quad {replaced}\quad {transition}\quad {metal}\quad {ion}\quad M} \pm 0.5}},$

E_(i) is the oxidation state of dopant B_(i) in the final productLiM_(y-x){A}_(x)O_(z) or LiM_(y-x){A}_(x)O_(z), the dopant elementsB_(i) are selected to be cations in the intercalation compound, at leasttwo of the dopant elements B_(i) have a different oxidation state thanthe oxidation state of M in the LiM_(y-x){A}_(x)O_(z) orM_(y-x){A}_(x)O_(z) compound, the dopant elements B_(i) include Ti⁴⁺ andMg²⁺, y and z are values that provide a stable metal oxide compound; andthe ratio of Li to O in the doped intercalation compound is not smallerthan the ratio of Li to O in the undoped compound LiM_(y)O_(z) orM_(y)O_(z); firing the mixture at a temperature between 500° C. and1000° C. in the presence of oxygen to produce the doped intercalationcompound; and cooling the doped intercalation compound.
 12. The methodof claim 11, wherein the step of mixing source compounds comprisingmixing source compounds containing a transition metal M selected fromCo, Ni, Mn, Fe, V and Mo.
 13. The method of claim 11, wherein the stepof mixing source compounds comprising mixing source compounds containingdopant elements Bi other than M having a Pauling's electronegativity notgreater than 2.05 or Mo.
 14. The method of claim 11, wherein the step ofmixing source compounds comprising mixing source compounds containingdopant elements Bi wherein the dopant elements B_(i) include no morethan one element from Groups 111B and IVB.
 15. The method of claim 11,wherein the step of mixing source compounds comprising mixing sourcecompounds containing Ni or Co as the transition metal M to form anintercalation compound having the formula LiMY-X{A}_(x)O_(z).
 16. Themethod of claim 15, wherein the step of mixing source compoundscomprising mixing source compounds containing Ni as the transition metalM.
 17. The method of claim 11, wherein the step of mixing sourcecompounds comprises preparing a solution comprising M and {A} fromsource compounds comprising M and {A}, precipitating the M and {A} outof solution to produce an intimately mixed hydroxide, and blending themixed hydroxide with a lithium source compound.